That tool sis completely the opinion of its author and not based on any actual formulas used by the search engines. I wouldn't put too much weight into its results.
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John Conde♦Nov 4 '10 at 15:15

Open Site Explorer is built by SEOmoz and is an approximation of how search engines view sites based on various criteria. The tool set has been in use by SEOmoz Pro members for years (myself included) and while nothing in this industry is perfect it's a solid steering mechanism. You can increase your page authority through links from sites with high trust and authority.
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AJ KohnNov 4 '10 at 15:47

Penguin has tossed Page Authority on its head. One of my competitors has a PA of 71 (compared to my 58) but has pretty much disappeared from the Google's index. Probably due to bad link building which boosted their PA.
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user16708Jul 2 '12 at 23:53

2 Answers
2

Open Site Explorer is an independent tool from SEOmoz, so take any data with a pinch of salt. OSE can be a good guide to compare sites but it's far from an authoritative source; search engine rankings will not always correlate to values from OSE.

Page Authority is simply their guess at how important a webpage is. It's generally based on links pointing to the page in a similar vein to PageRank. There is a video explaining it here.